The invention relates to projection lithography employing soft x-rays and in particular to a lithographic system including a multi-chamber housing the reticle, optics, e.g., camera, and wafer zones. The zones are vibrationally isolated and maintained at different pressures with the aid of conductance limiting seals.
In general, lithography refers to processes for pattern transfer between various media. A lithographic coating is generally a radiation-sensitized coating suitable for receiving a projected image of the subject pattern. Once the image is projected, it is indelibly formed in the coating. The projected image may be either a negative or a positive of the subject pattern. Typically, a xe2x80x9ctransparencyxe2x80x9d of the subject pattern is made having areas which are selectively transparent, opaque, reflective, or non-reflective to the xe2x80x9cprojectingxe2x80x9d radiation. Exposure of the coating through the transparency causes the image area to become selectively crosslinked and consequently either more or less soluble (depending on the coating) in a particular solvent developer. The more soluble (i.e., uncrosslinked) areas are removed in the developing process to leave the pattern image in the coating as less soluble crosslinked polymer.
Projection lithography is a powerful and essential tool for microelectronics processing. As feature sizes are driven smaller and smaller, optical systems are approaching their limits caused by the wavelengths of the optical radiation. xe2x80x9cLongxe2x80x9d or xe2x80x9csoftxe2x80x9d x-rays (a.k.a. Extreme UV) (wavelength range of xcex=100 to 200 xc3x85 (xe2x80x9cAngstromxe2x80x9d)) are now at the forefront of research in efforts to achieve the smaller desired feature sizes. Soft x-ray radiation, however, has its own problems. The complicated and precise optical lens systems used in conventional projection lithography do not work well for a variety of reasons. Chief among them is the fact that there are no transparent, non-absorbing lens materials for soft x-rays and most x-ray reflectors have efficiencies of only about 70%, which in itself dictates very simple beam guiding optics with very few surfaces.
One approach has been to develop cameras that use only a few surfaces and can image with acuity (i.e., sharpness of sense perception) only along a narrow arc or ringfield. Such cameras then scan a reflective mask across the ringfield and translate the image onto a scanned wafer for processing. Although cameras have been designed for ringfield scanning, e.g., Jewell et al., U.S. Pat. No. 5,315,629 and Offner, U.S. Pat. No. 3,748,015, available condensers that can efficiently couple the light from a synchrotron source to the ringfield required by this type of camera have not been fully explored. Furthermore, fill field imaging, as opposed to ringfield imaging, requires severely aspheric mirrors in the camera. Such mirrors cannot be manufactured to the necessary tolerances with present technology for use at the required wavelengths.
The present state-of-the-art for Very Large Scale Integration (xe2x80x9cVLSIxe2x80x9d) involves chips with circuitry built to design rules of 0.25 xcexcm. Effort directed to further miniaturization takes the initial form of more fully utilizing the resolution capability of presently-used ultraviolet (xe2x80x9cUVxe2x80x9d) delineating radiation. xe2x80x9cDeep UVxe2x80x9d (wavelength range of xcex=0.3 xcexcm to 0.1 xcexcm), with techniques such as phase masking, off-axis illumination, and step-and-repeat may permit design rules (minimum feature or space dimension) of 0.18 xcexcm or slightly smaller.
To achieve still smaller design rules, a different form of delineating radiation is required to avoid wavelength-related resolution limits. One research path is to utilize electron or other charged-particle radiation. Use of electromagnetic radiation for this purpose will require x-ray wavelengths.
Two x-ray radiation sources are under consideration. One source, a plasma x-ray source, depends upon a high power, pulsed laser (e.g., a yttrium aluminum garnet (xe2x80x9cYAGxe2x80x9d) laser), or an excimer laser, delivering 500 to 1,000 watts of power to a 50 xcexcm to 250 xcexcm spot, thereby heating a source. material to, for example, 250,000xc2x0 C., to emit x-ray radiation from the resulting plasma. Plasma sources are compact, and may be dedicated to a single production line (so that malfunction does not close down the entire plant). Another source, the electron storage ring synchrotron, has been used for many years and is at an advanced stage of development. Synchrotrons are particularly promising sources of x-rays for lithography because they provide very stable and defined sources of x-rays.
A variety of x-ray patterning approaches are under study. Probably the most developed form of x-ray lithography is proximity printing. In proximity printing, object:image size ratio is necessarily limited to a 1:1 ratio and is produced much in the manner of photographic contact printing. A fine-membrane mask is maintained at one or a few microns spacing from the wafer (i.e., out of contact with the wafer, thus, the term xe2x80x9cproximityxe2x80x9d), which lessens the likelihood of mask damage but does not eliminate it. Making perfect masks on a fragile membrane continues to be a major problem. Necessary absence of optics in-between the mask and the wafer necessitates a high level of parallelism (or collimation) in the incident radiation. X-ray radiation of wavelength xcexxe2x89xa616 xc3x85 is required for 0.25 xcexcm or smaller patterning to limit diffraction at feature edges on the mask.
Projection lithography has natural advantages over proximity printing. One advantage is that the likelihood of mask damage is reduced, which reduces the cost of the now larger-feature mask. Imaging or camera optics in-between the mask and the wafer compensate for edge scattering and, so, permit use of longer wavelength radiation. Use of extreme ultra-violet radiation (a.k.a., soft x-rays) increases the permitted angle of incidence for glancing-angle optics. The resulting system is known as extreme UV (xe2x80x9cEUVLxe2x80x9d) lithography (a.k.a., soft x-ray projection lithography (xe2x80x9cSXPLxe2x80x9d)).
A favored form of EUVL is ringfield scanning. All ringfield optical forms are based on radial dependence of aberration and use the technique of balancing low order aberrations, i.e., third order aberrations, with higher order aberrations to create long, narrow illumination fields or annular regions of correction away from the optical axis of the system (regions of constant radius, rotationally symmetric with respect to the axis). Consequently, the shape of the corrected region is an arcuate or curved strip rather than a straight strip. The arcuate strip is a segment of the circular ring with its center of revolution at the optic axis of the camera. See FIG. 4 of U.S. Pat. No. 5,315,629 for an exemplary schematic representation of an arcuate slit defined by width, W, and length, L, and depicted as a portion of a ringfield defined by radial dimension, R, spanning the distance from an optic axis and the center of the arcuate slit. The strip width is a function of the smallest feature to be printed with increasing residual astigmatism, distortion, and Petzval curvature at distances greater or smaller than the design radius being of greater consequence for greater resolution. Use of such an arcuate field allows minimization of radially-dependent image aberrations in the image. Use of object:image size reduction of, for example, 5:1 reduction, results in significant cost reduction of the, now, enlarged-feature mask.
It is expected that effort toward adaptation of electron storage ring synchrotron sources for EUVL will continue. Economical high-throughput fabrication of 0.25 xcexcm or smaller design-rule devices is made possible by use of synchrotron-derived x-ray delineating radiation. Large angle collection over at least 100 mrad will be important for device fabrication. Design of collection and processing optics for the condenser is complicated by the severe mismatch between the synchrotron light emission pattern and that of the ringfield scan line.
Aside from the quality of the optics that are employed in EUVL systems, factors that influence the quality of the printed wafers fabricated include the ability of the systems to prevent contaminants from depositing onto the surfaces of lens and mirrors and other optical devices. A possible source of contaminants are the hydrocarbons generated by the wafer upon exposure to radiation. Reducing the amount of such deposits will enhance overall quality and performance. Another factor that will affect the quality of the printed wafer is the ability of projection photolithography systems to be vibrationally isolated.
The invention is based in part on the recognition that control of particle contamination on the reticle and carbon contamination of optical surfaces in photolithography systems can be achieved by the establishment of multiple pressure zones in the photolithography systems. The different zones will enclose the reticle, projection optics, wafer, and other components of system.
Accordingly, in one aspect, the invention is directed to a seal assembly for connecting first and second members that are spaced apart and for providing a conductance limiting path, the seal assembly including:
a support that is attached to the first member;
means for adjusting the height of the support, wherein the support and the second member define an aperture; and
means for sealing the aperture.
In another aspect of the invention is directed to a vacuum apparatus that includes:
housing defining a vacuum chamber;
a tray situated within the vacuum chamber which is supported by at least one support member, wherein the tray separates the vacuum chamber into a first compartment and a second compartment which are at different pressures; and
means for adjoining the perimeter of the tray to an inner surface of the housing wherein the tray is decoupled from vibrations emanating from the housing and wherein the means for adjoining the perimeter of the tray comprises a conductance limiting seal.
In a further aspect, the invention is directed to a vacuum apparatus that includes:
a housing having an outer enclosure that defines a cavity and an inner enclosure that defines a vacuum chamber wherein the inner enclosure is positioned with the cavity;
a tray situated within the vacuum chamber which is supported by at least one support member, wherein the tray separates the vacuum chamber into a first compartment and a second compartment which are at different pressures;
means for adjoining the perimeter of the tray to an inner surface of the inner enclosure wherein the tray is decoupled from vibrations emanating from the inner enclosure; and
means for releasably attaching the inner enclosure to outer inner enclosure wherein the inner enclosure is decoupled from vibrations emanating from the outer enclosure.
In yet another aspect, the invention is directed to a photolithography system that includes:
a housing having an outer enclosure that defines a cavity and an inner enclosure that defines a vacuum chamber wherein the inner enclosure is positioned with the cavity;
a first tray situated within the vacuum chamber which is supported by at least one first support member, wherein the first tray separates the vacuum chamber into a first compartment and a second compartment which are at different pressures;
a second tray that is spaced apart from the first tray and which is situated within the vacuum chamber and which is supported by at least one second support member, wherein the second tray defines a third chamber that is at a different pressure from that of the first chamber and second chamber;
means for adjoining the perimeter of the first tray to a first inner surface of the inner enclosure wherein the first tray is decoupled from vibrations emanating from the inner enclosure;
means for adjoining the perimeter of the second tray to a second inner surface of the inner enclosure wherein the second tray is decoupled from vibrations emanating from the inner enclosure; and
means for releasably attaching the inner enclosure to the outer enclosure wherein the inner enclosure is decoupled from vibrations emanating from the outer enclosure.
In a yet another aspect, the invention is directed to a photolithography system that includes:
a housing having an outer enclosure that defines a cavity and an inner enclosure that defines a vacuum chamber wherein the inner enclosure is positioned with the cavity;
a first tray situated within the vacuum chamber which is supported by at least one first support member, wherein the first tray separates the vacuum chamber into a first compartment and a second compartment which are at different pressures;
a second tray that is spaced apart from the first tray and which is situated within the vacuum chamber and which is supported by at least one second support member, wherein the second tray defines a third chamber that is at a different pressure from that of the first chamber and second chamber;
a reticle stage positioned within the first chamber that supports a reflective reticle;
a wafer stage positioned within the third chamber that supports a wafer;
a projection optics device positioned in the second chamber that projects extreme ultraviolet radiation toward the reflective reticle;
a camera that collects extreme radiation reflected from the reflective reticle and directing the radiation toward the wafer;
means for adjoining the perimeter of the first tray to a first inner surface of the inner enclosure wherein the first tray is decoupled from vibrations emanating from the inner enclosure;
means for adjoining the perimeter of the second tray to a second inner surface of the inner enclosure wherein the second tray is decoupled from vibrations emanating from the inner enclosure; and
means for releasably attaching the inner enclosure to the outer enclosure wherein the inner enclosure is decoupled from vibrations emanating from the outer enclosure.
In a preferred embodiment, the means for adjoining the perimeter of the first tray comprises a conductance limiting seal and the means for adjoining the perimeter of the second tray comprises a conductance limiting seal. In addition, the means for releasably attaching the outer enclosure comprises a conductance limiting seal. The trays typically are metrology trays supporting various instruments integral to controlling and operating the photolithography process. The metrology trays in effect function as dividing planes in vacuum chamber.